(1) Field of the Invention
The present invention relates to a multiple speaker which is of piezoelectric type in its mode of drive and which is of cone type in its mode of radiation.
(2) Description of the Prior Art
The piezoelectric speaker is known as a high-frequency sound reproducing speaker. FIG. 1 is a schematic illustration in section of a piezoelectric speaker of conventional type, which has a disc-shaped piezoelectric element 91 comprising a metal base 911 and a piezoelectric material 912 bonded thereto. The element 91 is flexed when acoustic signals are applied between the metal base 911 and electrodes (not shown) disposed on the face of the piezoelectric material 912. A speaker diaphragm (paper cone) 92 having its smaller opening connected to the face of the piezoelectric element 91 at the center thereof generates sound when it is subjected to vibration due to flexing of the piezoelectric element 91. The larger opening of the speaker diaphragm 92 is peripherally secured to the peripheral edge of a speaker frame 93 having a saucer shape. A cushioning member 94 such as urethane foam is interposed between the underside of the piezoelectric element 91 or metal base 911 and the bottom of the frame 93 so that the transducing unit consisting of piezoelectric element 91 and speaker diaphragm 92 is softly held in position by the frame 93.
FIG. 2 shows by way of example the frequency response of a conventional piezoelectric speaker having the above described construction and whose piezoelectric element conforms to the following specifications:
Outer diameter: 21 mm PA1 Thickness: 230 .mu.m PA1 Resonant frequency: 3.85 kHz PA1 Resonant resistance: 150.OMEGA. or below PA1 Electrostatic capacity: 90 nF
The outer diameter of the cone (at its larger opening) is 40 mm.
As can be clearly seen from the figure, the frequency range available for sound reproduction is generally narrow. In order to obtain sound reproduction of a wider range, therefore, it is necessary to employ a plurality of piezoelectric speakers with their respective piezoelectric elements different in diameter from one another. This may be explained by the fact that a reproducing frequency range is governed by the resonant frequency of the element, while said resonant frequency is inversely proportional to the diameter of the element.